The present invention pertains to furnaces, and more particularly to a microprocessor control system and method for providing high and low heating rates in a furnace utilizing a single stage thermostat.
Presently, many furnace controls include a two-stage thermostat for providing dual rate heating, wherein the first stage of the thermostat operates the furnace on low heat and the second stage of the thermostat operates the furnace on high heat. For example, a typical two-stage thermostat comprises two small mercury bulb contacts on a bi-metal sensor that close and open as a function of the movement of the bi-metal sensor in response to changing room temperatures. If the present room temperature is below a desired temperature set point, for example, one degree to two degrees below the set point, then the low mercury heat bulb contact closes to provide low heat. If the room temperature continues to drop to, for example, three degrees below the desired temperature set point, then the second mercury bulb contact closes to provide a high rate of heat. When the heating load is satisfied, both of the mercury bulb contacts in the thermostat will sequentially open or deactivate, thereby terminating the heating cycle. Thereafter, when the thermostat contacts close indicating a new heating load to be satisfied, the furnace control repeats the same identical heating cycle.
Disadvantages of the above type of furnace control include the requirement of a two-stage thermostat for providing dual rates of heat and the existence of large swings in room temperature.
In an earlier patent, U.S. Pat. No. 4,638,942 to Ballard et al., assigned to a common assignee, an adaptive microprocessor control system and method was taught which overcame the aforementioned disadvantages.
In addition to the benefits achieved by the Ballard et al. patent, however, it is desirable to have a furnace which recovers quickly from setback such as, in particular night-setback. It is also desirable to reduce off-cycle electrical power draw, which reduction can result in savings on the average cost of operation of the furnace.